University of Basel > Condensed Matter Theory
Eugene Sukhorukov, Dr.
Address:  Département de Physique
Théorique

email:  Eugene.Sukhorukov@physics.unige.ch 
office:  +41 (0)22 702 6374 
fax:  +41 (0)22 702 6870 
Noise of a QuantumDot System in the
Cotunneling Regime
condmat/0010458
We study the noise of the cotunneling current through one or
several tunnelcoupled quantum dots in the Coulomb blockade
regime. The various regimes of weak and strong, elastic and
inelastic cotunneling are analyzed for quantumdot systems (QDS)
with fewlevel, nearlydegenerate, and continuous electronic
spectra. We find that in contrast to sequential tunneling where
the noise is either Poissonian (due to uncorrelated tunneling
events) or subPoissonian (suppressed by charge conservation on
the QDS), the noise in inelastic cotunneling can be superPoissonian
due to switching between QDS states carrying currents of
different strengths. In the case of weak cotunneling we prove a
nonequilibrium fluctuationdissipation theorem which leads to a
universal expression for the noisetocurrent ratio (Fano factor).
In order to investigate strong cotunneling we develop a
microscopic theory of cotunneling based on the densityoperator
formalism and using the projection operator technique. The master
equation for the QDS and the expressions for current and noise in
cotunneling in terms of the stationary state of the QDS are
derived and applied to QDS with a nearly degenerate and
continuous spectrum.
AndreevTunneling, Coulomb Blockade, and
Resonant Transport of NonLocal SpinEntangled Electrons
condmat/0009452
We propose and analyze a spinentangler for electrons based on an
swave superconductor coupled to two quantum dots each of which
is tunnelcoupled to normal Fermi leads. We show that in the
presence of a voltage bias and in the Coulomb blockade regime two
correlated electrons provided by the Andreev process can
coherently tunnel from the superconductor via different dots into
different leads. The spinsinglet coming from the Cooper pair
remains preserved in this process, and the setup provides a
source of mobile and nonlocal spinentangled electrons. The
transport current is calculated and shown to be governed by a twoparticle
BreitWigner resonance which allows the injection of two spinentangled
electrons into different leads at exactly the same orbital
energy, which is a crucial requirement for the detection of spin
entanglement via noise measurements.The coherent tunneling of
both electrons into the same lead is suppressed by the onsite
Coulomb repulsion and/or the supercondcuting gap, while the
tunneling into different leads is suppressed through the initial
separation of the tunneling electrons. In the regime of interest
the particlehole excitations of the leads are shown to be
negligible. The AharonovBohm oscillations in the current are
shown to contain single and twoelectron periods with amplitudes
that both vanish with increasing Coulomb repulsion albeit
differently fast.
Shot Noise by Quantum Scattering in
Chaotic Cavities
condmat/0009087
We have experimentally studied shot noise of chaotic cavities
defined by two quantum point contacts in series. The cavity noise
is determined as 1/4*2eI in agreement with theory and can be
well distinguished from other contributions to noise generated at
the contacts. Subsequently, we have found that cavity noise
decreases if one of the contacts is further opened and reaches
nearly zero for a highly asymmetric cavity.
Quantum Dot as Spin Filter and Spin
Memory
Phys. Rev.
Lett. 85 19621965 (2000); condmat/0003089
We consider a quantum dot in the Coulomb blockade regime weakly
coupled to current leads and show that in the presence of a
magnetic field the dot acts as an efficient spinfilter (at the
singlespin level) which produces a spinpolarized current.
Conversely, if the leads are fully spinpolarized the up or down
state of the spin on the dot results in a large sequential or
small cotunneling current, and thus, together with ESR
techniques, the setup can be operated as a singlespin memory.
Quantum Computation and Spin Electronics
See condmat/9911245.
Published in Quantum Mesoscopic Phenomena and Mesoscopic Devices
in Microelectronics,
ed. I. O. Kulik (NATO Advanced Study Institute, Turkey, June 1325,
1999).
In this chapter we explore the connection between mesoscopic
physics and quantum computing. After giving a bibliography
providing a general introduction to the subject of quantum
information processing, we review the various approaches that are
being considered for the experimental implementation of quantum
computing and quantum communication in atomic physics, quantum
optics, nuclear magnetic resonance, superconductivity, and,
especially, normalelectron solid state physics. We discuss
five criteria for the realization of a quantum computer and
consider the implications that these criteria have for quantum
computation using the spin states of singleelectron quantum dots.
Finally, we consider the transport of quantum information via the
motion of individual electrons in mesoscopic structures; specific
transport and noise measurements in coupled quantum dot
geometries for detecting and characterizing electronstate
entanglement are analyzed.
Transport and Noise of Entangled
Electrons
Proceedings of the XVI Sitges Conference (Lecture Notes in
Physics, Springer), see condmat/9909348.
We consider a scattering setup with an entangler and beam
splitter where the current noise exhibits bunching behavior for
electronic singlet states and antibunching behavior for triplet
states. We show that the entanglement of two electrons in the
doubledot can be detected in mesoscopic transport measurements.
In the cotunneling regime the singlet and triplet states lead to
phasecoherent current contributions of opposite signs and to
AharonovBohm and Berry phase oscillations in response to
magnetic fields. We analyze the Fermi liquid effects in the
transport of entangled electrons.
Quantum Computing and Quantum
Communication with Electrons in Nanostructures
Proceedings of the XXXIVth Rencontres de Moriond "Quantum
Physics at Mesoscopic Scale", held in Les Arcs, Savoie,
France, January 2330, 1999; see condmat/9907133.
Addressing the feasibilty of quantum communication with electrons
we consider entangled spin states of electrons in a doubledot
which is weakly coupled to inand outgoing leads. We show that
the entanglement of two electrons in the doubledot
can be detected in mesoscopic transport and noise measurements.
In the Coulomb blockade and cotunneling regime the singlet and
triplet states lead to phasecoherent current and noise
contributions of opposite signs and to AharonovBohm and
Berry phase oscillations in response to magnetic fields. These
oscillations are a genuine twoparticle effect and provide a
direct measure of nonlocality in entangled
states. We show that the ratio of zerofrequency noise to current
(Fano factor) is universal and equal to the electron charge.
Probing Entanglement and Nonlocality of
Electrons in a DoubleDot via Transport and Noise
Phys. Rev.
Lett. 84 10351038 (2000); condmat/9907129.
Addressing the feasibilty of quantum communication with electrons
we consider entangled spin states of electrons in a doubledot
which is weakly coupled to inand outgoing leads. We show that
the entanglement of two electrons in the doubledot can be
detected in mesoscopic transport and noise measurements. In the
Coulomb blockade and cotunneling regime the singlet and triplet
states lead to phasecoherent current and noise contributions of
opposite signs and to AharonovBohm and Berry phase oscillations
in response to magnetic fields. These oscillations are a genuine
twoparticle effect and provide a direct measure of nonlocality
in entangled states. We show that the ratio of zerofrequency
noise to current (Fano factor) is universal and equal to the
electron charge.
Noise of Entangled Electrons: Bunching
and Antibunching
Phys.
Rev. B 61 1630316306 (2000); condmat/9906071.
Addressing the feasibility of quantum communication with
entangled electrons we consider a scattering setup with an
entangler and beam splitter where the current noise exhibits
bunching behavior for electronic singlet states and antibunching
behavior for triplet states. The Fano factor (noisetocurrent
ratio) is calculated and shown to contain opposite signs for
singlets and triplets. We further show that spin currents can
produce noise even in the absence of any charge currents.
Semiclassical Theory of Conductance and
Noise in Open Chaotic Cavities
Phys. Rev.
Lett. 84 12801283 (2000); see condmat/9904448.
Conductance and shot noise of an open cavity with diffusive boundary scattering are calculated within the BoltzmannLangevin approach. In particular, conductance contains a nonuniversal geometric contribution, originating from the presence of open contacts. Subsequently, universal expressions for multiterminal conductance and noise valid for all chaotic cavities are obtained classically basing on the fact that the distribution function in the cavity depends only on energy and using the principle of minimal correlations.
Noise in Multiterminal Diffusive
Conductors: Universality, Nonlocality and Exchange Effects
Phys. Rev.
Lett. 80, 4959 (1998), see condmat/9802050;
Phys.
Rev. B 59, 1305413066 (1999), see condmat/9809239.
We study noise and transport in multiterminal diffusive
conductors. Using a BoltzmannLangevin equation approach we
reduce the calculation of shotnoise correlators to the solution
of diffusion equations. Within this approach we prove the
universality of shot noise in multiterminal diffusive conductors
of arbitrary shape and dimension for purely elastic scattering as
well as for hot electrons. We show that shot noise in
multiterminal conductors is a nonlocal quantity and that
exchange effects can occur in the absence of quantum phase
coherence even at zero electron temperature. It is also shown
that the exchange effect measured in one contact is always
negative  in agreement with the Pauli principle. We discuss a
new phenomenon in which current noise is induced by thermal
transport. We propose a possible experiment to measure locally
the effective noise temperature. Concrete numbers for shot noise
are given that can be tested experimentally.
Magneticfielddependent Zerobias
Diffusive Anomaly in Pb OxidenInAs Structures: Coexistence of
Two and Threedimensional States
Phys.
Rev. B 59 1313913146 (1999), see condmat/9807041.
The results of experimental and theoretical studies of zerobias anomaly (ZBA) in the PboxidenInAs tunnel structures in magnetic field up to 6T are presented. A specific feature of the structures is a coexistence of the 2D and 3D states at the Fermi energy near the semiconductor surface. The dependence of the measured ZBA amplitude on the strength and orientation of the applied magnetic field is in agreement with the proposed theoretical model. According to this model, electrons tunnel into 2D states, and move diffusively in the 2D layer, whereas the main contribution to the screening comes from 3D electrons.